JP2020002416A - Manufacturing method of cold-rolled steel sheet, hot-dip galvanized steel sheet and galvanized steel sheet - Google Patents

Abstract

To provide a method of manufacturing a cold-rolled steel sheet having excellent formability and bake hardenability.SOLUTION: In the method of manufacturing the cold-rolled steel sheet that includes a hot-rolling step for hot-rolling a steel material to obtain a hot-rolled steel sheet, a cold-rolling step for cold-rolling to obtain a cold-rolled steel sheet, and an annealing step for annealing to obtain an annealed cold-rolled steel sheet, the steel material contains, in mass%, C:0.0010% or more and 0.0030% or less, Nb:0.010% or more and 0.025% or less, Al:0.050% or more and 0.080% or less, Si:0.05% or less, Mn:1.0% or less, P:0.10% or less, S:0.010% or less, B:0.0030% or less, N: 0.0030% or less, and balance consisting of Fe and unavoidable impurities, and has a composition that satisfies the formula(1), (Al/N)+(100×B/N)>100 (1). At the annealing step, a temperature is kept at 850°C or more and less than 910°C, and subsequently cooling is performed so as to cool to 750°C or less at an average cooling rate of 5°C/s or more.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet.

In recent years, exhaust gas regulations have been implemented to reduce pollutants emitted from the viewpoint of preserving the global environment, and there is a strong demand for automobiles to improve fuel efficiency by reducing the weight of vehicle bodies. One of the leading methods for reducing the weight of a vehicle body is to increase the strength of a thin steel sheet used, and the amount of high-strength steel sheet used is increasing year by year.
As a method for increasing the strength of thin steel sheets, a method using solid solution strengthening by adding a solid solution strengthening element, a method using a precipitate by adding a precipitation strengthening element, and a method using martensite which is a hard phase Generally, a strengthening method is adopted. However, each of these methods has a problem in that, as the strength is increased, the formability and spot weldability are deteriorated due to the decrease in ductility and r-value, and the secondary work brittleness resistance is lowered. For this reason, when manufacturing a high-strength steel sheet excellent in formability, the method of adding the above strengthening element or the method of using martensite which is a hard phase is not necessarily a preferable method.
Another method for reducing the weight of a vehicle body is to improve the formability of a thin steel plate. By improving the formability of thin steel sheets, it is possible to change from the conventional welding assembly method, which welds and assembles many parts, to an integrated molding method. Becomes possible. In addition, the integral molding has an advantage that the manufacturing cost can be reduced in addition to the weight reduction.
As a measure to achieve both high moldability and high strength, it is soft at the time of molding, and achieves high strength in the final part (product) after assembly painting by processing distortion introduced at the time of molding and subsequent baking treatment There is a way to do it. A steel sheet using this method is called a bake hardening steel sheet. By using this method, at the time of forming, it is soft and has good formability, while, on the other hand, utilizing the so-called bake hardening phenomenon, which is the strain aging phenomenon of steel caused by the work strain introduced into the steel sheet and the paint baking treatment after forming. The deformation strength can be increased, and the strength of the final part (product) can be increased. Although this method has a limitation in the amount of increase in strength, it is soft when formed into the shape of the final part (product) and has the advantage that the final part (product) can be strengthened after baking. There is. In addition, this method has an advantage that the spot weldability is good because no strengthening element is added, and the presence of solid solution C that increases the grain boundary strength does not easily cause a reduction in the secondary work brittleness resistance. In addition, since no additional element is used, there is an advantage that manufacturing cost can be reduced.
Several proposals have been made so far, for example, in Patent Document 1 and Patent Document 2, with respect to a method for producing a bake hardenable steel sheet having such advantages.

  Patent Document 1 contains C: 0.0005 to 0.0035% by mass, and Nb: 5C to 9C% by mass (where C is the amount of C (% by mass)) so as to satisfy a specific relationship. After hot rolling and cold rolling are performed on a steel material containing specific amounts of Si, Mn, B, Al, and N, continuous annealing is performed from 730 ° C. to a specific temperature T determined by the amounts of Nb and C. A method for producing a thin steel sheet having high bake hardenability and surface roughening resistance, characterized in that the warming time is 30 seconds or more and the stay time at a specific temperature T or more is 40 seconds or more, is disclosed. In Patent Document 1, the timing of the progress of recrystallization and the dissolution of Nb carbide can be appropriately adjusted by the above method, a high BH amount (bake hardening amount) can be ensured, and the improvement of rough surface resistance and deep drawing can be achieved. The moldability can be improved.

  Further, in Patent Document 2, C: 0.0008 to 0.0025% by mass, Nb: 0.008 to 0.020% by mass, Excess C amount = C− (12/93) Nb> −0.0005% After hot rolling, pickling and cold rolling are performed on a steel material containing specific amounts of Si, Mn, P, S, Al and B, the hot-dip galvanized zinc line The annealing temperature at 800 ° C. or more and less than 850 ° C. when the amount of Excess C exceeds 0%, and 850 ° C. or more and less than the Ac3 transformation point when the amount of Excess C is −0.0005% or more and 0% or less, Further, there is disclosed a method for producing a galvanized steel sheet having excellent bake hardenability, wherein the steel sheet is cooled to a temperature of 750 ° C. or less at an average cooling rate of 5 ° C./s or more after annealing at the annealing temperature. . Patent Literature 2 discloses that the above method can appropriately adjust the dissolution temperature of Nb carbide and ensure a high BH amount (bake hardening amount).

JP-A-8-100221 JP 2007-270167 A JP-A-3-281732

However, the present inventors have studied the techniques described in Patent Literature 1 and Patent Literature 2, and found that, for example, the BH amount (the amount of bake hardening) greatly fluctuates due to variations in the amounts of C and Nb during tapping. Therefore, although the ductility is excellent, the BH amount (bake hardening amount) is lower than the target value and cannot satisfy the characteristics required by the user, or conversely, the BH amount (bake hardening amount) is high, but the ductility is reduced, and press cracking occurs. It became clear that problems, such as easy occurrence, may occur. In addition, it has also been clarified that, due to variations in the amounts of C and Nb at the time of tapping, stretchers and strains are likely to be generated during press working and the surface appearance may be significantly impaired.
Further, when a steel sheet obtained by the method described in Patent Document 3 was examined, it became clear that the formability and the BH amount (bake hardening amount) did not always satisfy the standards required recently.

  In view of the above circumstances, an object of the present invention is to provide a method for producing a cold-rolled steel sheet having excellent formability and bake hardenability.

The present inventors have studied in detail the effects of steel composition and manufacturing conditions on the BH amount (bake hardening amount) in order to achieve the above object. As a result, while controlling so that C amount may be in the range of 0.0010% or more and 0.0030% or less and Nb amount in the range of 0.010% or more and 0.025% or less, the N amount may be 0.0030% or less. The amount of Al is set to 0.050% or more and 0.080% or less, and a predetermined Al, B, and N content balance is secured, and then the annealing temperature of the annealing treatment after cold rolling is adjusted, and the cooling rate in a specific temperature range is adjusted. By adjusting C to an appropriate range, it is possible to secure a stable BH amount (bake hardening amount) of 30 MPa or more by keeping the amount of solid solution C constant without precipitating C as a precipitate. Can be solved, and the present invention has been accomplished.
That is, the present inventors have found that the above problem can be solved by the following constitution.

(1) hot rolling a steel material to obtain a hot rolled steel sheet,
Cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, a cold rolling step,
Performing an annealing treatment on the cold-rolled steel sheet to obtain a cold-rolled steel sheet subjected to the annealing treatment, an annealing step,
A method for producing a cold-rolled steel sheet comprising:
The above steel material, in mass%,
C: 0.0010% or more and 0.0030% or less Nb: 0.010% or more and 0.025% or less Al: 0.050% or more and 0.080% or less Si: 0.05% or less Mn: 1.0% or less P: 0.10% or less S: 0.010% or less B: 0.0030% or less N: 0.0030% or less, with the balance being Fe and unavoidable impurities, having a composition satisfying the following formula (1) Steel material,
(Al / N) + (100 × B / N)> 100 (1)
A method for producing a cold-rolled steel sheet, wherein the annealing treatment is a treatment of holding at a temperature of 850 ° C. or more and less than 910 ° C., and thereafter cooling to a temperature of 750 ° C. or less at an average cooling rate of 5 ° C./s or more.
(2) A method for producing a hot-dip galvanized steel sheet, wherein a hot-dip galvanized steel sheet is obtained by performing a hot-dip galvanizing treatment on the cold-rolled steel sheet after the annealing treatment obtained by the production method according to the above (1).
(3) A method for producing an alloyed hot-dip galvanized steel sheet, wherein an alloying treatment is performed on the hot-dip galvanized steel sheet obtained by the manufacturing method according to (2) to obtain an alloyed hot-dip galvanized steel sheet.

  As described below, according to the present invention, it is possible to provide a method for producing a cold-rolled steel sheet having excellent formability and bake hardenability.

Hereinafter, a method for producing a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet of the present invention will be described.
In this specification, a numerical range represented by using “to” means a range including numerical values described before and after “to” as a lower limit and an upper limit.

[Production method of cold rolled steel sheet]
The method for producing a cold-rolled steel sheet of the present invention (hereinafter, also referred to as “method of the present invention”)
Hot-rolling a steel material to obtain a hot-rolled steel sheet by hot rolling,
Cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, a cold rolling step,
Performing an annealing treatment on the cold-rolled steel sheet to obtain a cold-rolled steel sheet subjected to the annealing treatment, an annealing step,
A method for producing a cold-rolled steel sheet comprising:
The above steel material, in mass%,
C: 0.0010% or more and 0.0030% or less Nb: 0.010% or more and 0.025% or less Al: 0.050% or more and 0.080% or less Si: 0.05% or less Mn: 1.0% or less P: 0.10% or less S: 0.010% or less B: 0.0030% or less N: 0.0030% or less, with the balance being Fe and unavoidable impurities, having a composition satisfying the following formula (1) Steel material,
(Al / N) + (100 × B / N)> 100 (1)
This is a method for producing a cold-rolled steel sheet, wherein the above-mentioned annealing treatment is a treatment of holding at a temperature of 850 ° C or more and less than 910 ° C, and thereafter cooling to a temperature of 750 ° C or less at an average cooling rate of 5 ° C / s or more.

  Hereinafter, each step will be described.

(Hot rolling process)
The hot rolling step is a step of performing hot rolling on a steel material having a specific composition to obtain a hot-rolled steel sheet.

<Steel material>
Hereinafter, the composition of the steel material used in the hot rolling step will be described. The “%” indication in the composition means “% by mass” unless otherwise specified.

(C: 0.0010% or more and 0.0030% or less)
C is an element that increases the strength and develops bake hardenability, and it is desirable to add a large amount of C. However, when the added amount is large, ductility and moldability are reduced. In the present invention, the content of 0.0010% or more is required in order to secure a predetermined amount of solid solution C or more and obtain a bake hardening amount of 30 MPa or more. On the other hand, when the amount of addition increases, the decrease in ductility increases. For this reason, C is limited to 0.0030% or less. Preferably, it is in the range of 0.0020% or more and 0.0025% or less.

(Nb: 0.010% or more and 0.025% or less)
Nb is an element for improving the formability of the steel sheet and is an element for fixing C as a precipitate (carbide) before annealing. In order to obtain such effects, the content needs to be 0.010% or more. On the other hand, if it is contained excessively, the formability of the steel sheet deteriorates and C is excessively fixed as carbide, so that the carbide does not dissolve in the annealing process and the amount of solid solution C cannot be secured, resulting in baking hardening. Since the amount decreases, the content is limited to 0.025% or less. Preferably, it is in the range of 0.020% or more and 0.025% or less.

(Al: 0.050% to 0.080%)
Al acts as a deoxidizing agent in the steelmaking process and has a strong affinity with N, and has a strong tendency to form AlN. N in the steel sheet forms carbonitride by bonding with carbon, and the bake hardening amount is reduced. Therefore, it is better that the N in the steel sheet is small, so that N in the steel sheet is precipitated as AlN to secure the bake hardening amount. In order to do so, 0.050% or more is required. On the other hand, if it is contained excessively, it becomes coarse AlN, which causes a reduction in ductility of the steel sheet. Therefore, the content is limited to 0.080% or less. Preferably, it is in the range of 0.060% or more and 0.080% or less.

(Si: 0.05% or less)
Si is an effective element that acts as a deoxidizer in the steelmaking process and strengthens the steel. In order to obtain such an effect, it is desirable to contain 0.005% or more, but if it exceeds 0.05%, the moldability and the plating property are reduced. For this reason, Si was limited to 0.05% or less.

(Mn: 1.0% or less)
Mn is an effective element that acts as a deoxidizer in the steelmaking process and strengthens the steel. In order to obtain such an effect, it is desirable to contain 0.10% or more. Further, from the reason that the obtained cold-rolled steel sheet is more excellent in formability and bake hardenability (hereinafter also referred to as “the effect of the present invention is more excellent”), it is preferably more than 0.5%, and more preferably 0.6%. More preferably, it is more. On the other hand, an excessive content exceeding 1.0% increases the strength of the steel sheet more than necessary and makes the steel sheet brittle. For this reason, Mn was limited to 1.0% or less.

(P: 0.10% or less)
P has the effect of strengthening steel, and it is desirable to contain at least 0.02% or more in order to increase the strength. However, if it exceeds 0.10%, the secondary work brittleness resistance is reduced. Therefore, P is limited to 0.10% or less. From the reason that the effect of the present invention is more excellent, it is preferably 0.05% or less.

(S: 0.010% or less)
S is present as inclusions in steel and reduces formability. For this reason, although it is desirable to reduce S as much as possible, as the S is reduced, the manufacturing cost of the steel material increases. Therefore, the S content is set to 0.010% or less. In addition, it is preferably 0.005% or less.

(B: 0.0030% or less)
B is an element effective for fixing N because it combines with N to form BN, and is also an element effective for improving the resistance to secondary working embrittlement. In order to obtain such an effect, the content is preferably 0.0003% or more. On the other hand, when the content exceeds 0.0030%, the moldability decreases. For this reason, B is limited to the range of 0.0030% or less. In addition, from the reason that the effect of this invention is more excellent, Preferably it is 0.0020% or less.

(N: 0.0030% or less)
If N is contained in the steel sheet, a carbonitride is formed, and the amount of bake hardening is reduced. Therefore, it is desirable to reduce N as much as possible. Therefore, the N content is set to 0.0030% or less. If the content of N is 0.0030% or less, N combines with Al and B to form a nitride, so that the bake hardening amount can be secured. In addition, from the reason that the effect of this invention is more excellent, Preferably it is 0.0020% or less.

(The rest)
The balance other than the above components is Fe and unavoidable impurities.
In addition, O as an inevitable impurity is preferably 0.0050% or less for the reason that the effect of the present invention is more excellent.

(Equation (1))
The composition of the steel material used in the hot rolling step satisfies the following expression (1).
(Al / N) + (100 × B / N)> 100 (1)
Hereinafter, the left side of the equation (1) ((Al / N) + (100 × B / N)) is also referred to as “the left side of the equation (1)”.

  The left side of the formula (1) is preferably 110 or more, more preferably 120 or more, and still more preferably 130 or more, because the effect of the present invention is more excellent. The upper limit is not particularly limited, but is preferably 200 or less for the reason that the effect of the present invention is more excellent.

In addition, for example, if it is a steel material used for A in Table 1 in the Examples section described later, the left side of Expression (1) is calculated as follows.
Equation (1) of steel material used for A in Table 1 left side = (0.053 / 0.0013) + (100 × 0.0011 / 0.0013) ≒ 125
As described above, the left side of Formula (1) of the steel material used in A in Table 1 is approximately 125, and is more than 100, so the steel material used in A in Formula 1 satisfies Formula (1).

<Preferred embodiment>
A preferred embodiment of the hot rolling step includes, for example, a step of heating the above-mentioned steel material, then performing hot rolling, and winding the steel material.

(Heating temperature)
The heating temperature of the steel material is not particularly limited, but is preferably 1100 to 1250 ° C. because the effect of the present invention is more excellent.

(Rolling end temperature)
The rolling end temperature (FDT) of the hot rolling is preferably 900 ° C. or higher from the viewpoint of formability. The rolling end temperature is preferably 900 to 980 ° C. because the coarsening of the crystal grains of the hot-rolled steel sheet is suppressed, and as a result, the cold-rolled steel sheet obtained has better press workability (deep drawability).

(Winding temperature)
The winding temperature is preferably set to 700 ° C. or lower. When the winding temperature is higher than 700 ° C., the effect of improving the material is saturated and the pickling property is reduced. In addition, it is preferable that it is 500-700 degreeC from a viewpoint of promotion of precipitation of AlN and NbC, and improvement of the formability by coarsening of the said precipitate.

(Cold rolling process)
The cold rolling step is a step of performing cold rolling on the hot-rolled steel sheet obtained in the hot rolling step or the hot-rolled steel sheet after the pickling treatment described later to obtain a cold-rolled steel sheet.

<Reduction ratio>
Although the rolling reduction of the cold rolling is not particularly limited, it is preferably 50% or more because the effect of the present invention is more excellent.

[Annealing process]
The annealing step is a step of performing an annealing treatment on the cold-rolled steel sheet obtained in the cold rolling step to obtain a cold-rolled steel sheet that has been subjected to the annealing treatment.
Here, the annealing process is a process of holding at a temperature of 850 ° C. or more and less than 910 ° C., and thereafter cooling to a temperature of 750 ° C. or less at an average cooling rate of 5 ° C./s (second) or more.
The annealing is performed, for example, in a continuous annealing line. When the method of the present invention further includes a plating step described below, the annealing treatment is performed, for example, in a continuous galvanizing line.

<Annealing temperature>
As described above, the holding temperature (annealing temperature) of the annealing process is 850 ° C or more and less than 910 ° C.
In the annealing treatment, the precipitated NbC is dissolved to form solid solution C, and annealing is performed at 850 ° C. or more in order to secure a bake hardening amount of 30 MPa or more. On the other hand, when the annealing temperature becomes high, the precipitated AlN is dissolved by adding 0.050% or more of Al, and in the subsequent cooling process, carbonitride is formed by nitrogen dissolved in the solid solution, and the bake hardening amount is increased. Annealing is performed at a temperature of at most less than 910 ° C., since the risk of the decrease in the temperature increases.
In the annealing treatment, the above-mentioned holding at the annealing temperature is preferably set to about 50 to 300 s, because the effect of the present invention is more excellent.

<Average cooling rate and cooling stop temperature>
As described above, the steel sheet is held at a temperature of 850 ° C. or more and less than 910 ° C., and thereafter, the steel sheet is cooled to a temperature of 750 ° C. or less at an average cooling rate of 5 ° C./s or more. Here, the average cooling rate means an average from a cooling start temperature to a cooling stop temperature. If the average cooling rate is less than 5 ° C./s, C dissolved as a solid solution during the annealing treatment is reprecipitated as NbC, and the bake hardening amount is reduced. Therefore, a cooling rate of 5 ° C./s or more is required. If the cooling rate is too high, the ductility of the steel sheet will decrease, so a cooling rate of 10 ° C./s or less is preferable. Further, when the cooling stop temperature exceeds 750 ° C. and becomes high, solid solution C is re-precipitated as NbC in the subsequent slow cooling, and the bake hardening amount is reduced. Therefore, it is necessary to cool to 750 ° C. or less.

[Other steps]
The method of the present invention may include steps other than those described above.

<Pickling process>
The method of the present invention preferably further comprises a pickling treatment step, and more preferably further comprises a pickling treatment step after the hot rolling step and before the cold rolling step. When pickling is performed on the hot-rolled steel sheet after hot rolling, scale (oxide film) on the surface is removed, and scratches are less likely to occur during cold rolling, and the appearance is improved.

[Method of manufacturing hot-dip galvanized steel sheet]
The method for producing a hot-dip galvanized steel sheet according to the present invention comprises subjecting the cold-rolled steel sheet subjected to the annealing treatment obtained by the above-described method of the present invention to hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet. Is a manufacturing method.
The conditions for the hot-dip galvanizing treatment are not particularly limited. Any of the usual hot-dip galvanizing treatment conditions can be applied.

[Manufacturing method of galvannealed steel sheet]
The method for producing an alloyed hot-dip galvanized steel sheet according to the present invention includes subjecting a hot-dip galvanized steel sheet obtained by the above-described hot-dip galvanized steel sheet manufacturing method to an alloying treatment to obtain an alloyed hot-dip galvanized steel sheet. This is a method for producing a hot-dip galvanized steel sheet.

  Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited thereto.

[Manufacture of cold-rolled steel sheets, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets]
As described below, cold-rolled steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets were manufactured.

<Hot rolling process>
A slab (steel material) having a composition (mass%) shown in Table 1 was heated at a heating temperature of 1200 ° C., and then subjected to hot rolling to obtain a hot-rolled steel sheet having a thickness of 3.2 mm. In addition, the rolling end temperature of the hot rolling was 950 ° C., and the winding temperature was 680 ° C. After winding, the temperature was cooled to room temperature.

<Pickling process>
Next, the obtained hot-rolled steel sheet was subjected to an acid washing treatment.

<Cold rolling process>
Next, the hot-rolled steel sheet after the pickling treatment was subjected to cold rolling with a reduction of 75% to obtain a cold-rolled steel sheet having a thickness of 0.8 mm.

<Annealing process>
Next, the obtained cold-rolled steel sheet was subjected to an annealing treatment under the conditions shown in Table 2 using a continuous annealing line.

<Plating process>
Next, the steel sheet described as “Yes” in the column of “Plating treatment” in Table 2 was subjected to a hot-dip galvanizing treatment using a continuous hot-dip galvanizing line to obtain a hot-dip galvanized steel sheet.

<Alloying process>
Further, for the steel sheet described as “Yes” in the column of “Alloying treatment” in Table 2, alloying treatment was performed on the hot-dip galvanized layer to obtain an alloyed hot-dip galvanized steel sheet.

[Evaluation]
The following evaluation was performed about the obtained cold rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet.

<Tensile properties>
From the obtained cold-rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet, a JIS No. 5 tensile test piece having a long axis perpendicular to the rolling direction was sampled, and a tensile test was performed in accordance with JIS Z2241. And the tensile properties (yield strength, tensile strength, elongation) were evaluated.
Table 3 shows the results. For practical use, the elongation is preferably 40% or more.

<Moldability>
JIS No. 5 tensile test pieces were collected from the obtained cold-rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet, and the r value was measured. Specifically, after applying 15% pre-strain to the test piece by a tensile test, the r value was measured by the three-point method, and the rolling direction, the direction at 45 ° to the rolling direction, and the direction at 90 ° to the rolling direction. The r values in the direction of were designated as rL, rD and rC, respectively. Then, an average value (= (rL + 2rD + rC) / 4) was obtained.
Table 3 shows the results. The larger the r value, the better the moldability. In practice, it is preferably 1.50 or more.

<Bake hardenability>
From the obtained cold-rolled steel sheet, hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet, a JIS No. 5 tensile test piece was sampled in the same manner as in the evaluation of the tensile properties, and a 2% tensile test was conducted in accordance with the provisions of JIS G3135. A pre-deformation treatment for giving a pre-strain (plastic strain) is performed, and then a heat treatment is performed at 170 ° C. for 20 minutes to increase the deformation stress before and after the heat treatment (BH amount) (bake hardening amount) (= Yield point (MPa))-(pre-deformation stress before heat treatment (MPa)) was determined.
Table 3 shows the results. If the BH amount is 30 MPa or more, it can be said that bake hardenability is excellent.

In Table 1, the column of “Equation (1)” represents the value of the left side of the above-described Equation (1) ((Al / N) + (100 × B / N)). That is, if the value of the column of “Equation (1)” exceeds 100, it means that the above-described expression (1) is satisfied. If the value of the column of “Equation (1)” is 100 or less, the above-described expression is satisfied. It means that (1) is not satisfied.
In Tables 1 to 3, the underlined portions indicate out of the scope of the present invention.

  As can be seen from Tables 1 to 3, the steel sheets A to F obtained by subjecting a steel material having a specific composition to hot rolling and cold rolling and further performing an annealing treatment under specific conditions are 30MP or more. , And excellent moldability with an elongation of 40.0% or more and an r value of 1.50 or more. Above all, C and F in which the left side of the formula (1) is more than 130 showed more excellent bake hardenability.

  On the other hand, steel sheets of G to J and LM in which the steel material does not have a specific composition, and a steel sheet of K in which a steel material having a specific composition is used but the annealing conditions are not the specific conditions, At least one of the bake hardenability was insufficient.

Claims (3)

Hot-rolling a steel material to obtain a hot-rolled steel sheet by hot rolling,
Cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, a cold rolling step,
Performing an annealing treatment on the cold-rolled steel sheet to obtain a cold-rolled steel sheet subjected to the annealing treatment, an annealing step,
A method for producing a cold-rolled steel sheet comprising:
The steel material is expressed in mass%,
C: 0.0010% or more and 0.0030% or less Nb: 0.010% or more and 0.025% or less Al: 0.050% or more and 0.080% or less Si: 0.05% or less Mn: 1.0% or less P: 0.10% or less S: 0.010% or less B: 0.0030% or less N: 0.0030% or less, with the balance being Fe and unavoidable impurities, having a composition satisfying the following formula (1) Steel material,
(Al / N) + (100 × B / N)> 100 (1)
A method for producing a cold-rolled steel sheet, wherein the annealing treatment is a treatment of holding at a temperature of 850 ° C. or more and less than 910 ° C., and thereafter cooling to a temperature of 750 ° C. or less at an average cooling rate of 5 ° C./s or more.   A method for manufacturing a hot-dip galvanized steel sheet, wherein a hot-dip galvanized steel sheet is obtained by performing a hot-dip galvanizing treatment on the cold-rolled steel sheet after the annealing treatment obtained by the manufacturing method according to claim 1.   A method for manufacturing an alloyed hot-dip galvanized steel sheet, comprising subjecting a hot-dip galvanized steel sheet obtained by the manufacturing method according to claim 2 to alloying treatment to obtain an alloyed hot-dip galvanized steel sheet.